The present invention relates to inspecting an object of electrically conductive material. In the specification and in the claims the phrase "inspecting an object" is used to refer to operations such as measuring the thickness of the object, checking the object for the presence of sub-surface flaws and measuring the thickness of a layer of non-conductive material around the object. The object can be, for example, a wall of a container or the wall of a pipe.
The effective thickness of an object, for example, a wall of a steel container, a pipe or a vessel can be locally affected by external or internal corrosion. In case the object is provided with a layer of insulation material, corrosion detection by visual inspection generally implies temporary removal of the insulation material, which is time consuming and expensive.
U.S. Pat. No. 4,843,320 issued Jun. 27, 1989 to B. R. Spies discloses a device for measuring a thickness of an object of electrically conductive material, such as a wall, comprising a pulsed-signal transmitter for generating an electromagnetic field in the wall and a receiver for measuring the decay of the eddy current generated by the electromagnetic field and for producing a signal representing the decay.
To determine the thickness of the wall, the decay of the received signal over a period of time is compared with the decay of a reference signal indicative of a known wall thickness.
The transmitter includes an emitter coil and the receiver includes a receiver coil. The coils are wound around a core, and, during normal operation, the central longitudinal axis of the core is perpendicular to a surface of the object.
With the known device, corrosion spots of a size comparable to that of the emitter coil can be detected, however, it was found that spots which are relatively small compared to the coil size cannot be detected. Such small corrosion spots, however, can locally reduce the wall thickness to a significant extent. Furthermore, the size of such a small spot can rapidly grow, for example when liquid water is present between the pipe and an insulation layer surrounding the pipe.
It is an advantage of the invention to provide a device which provides an enhanced resolution so as to allow a more detailed inspection of the object.
It is another advantage of the invention to provide an improved method of interpreting measured data to determine wall thickness.